The growing incidence of obesity is placing an enormous burden on our health-care system and there is an urgent need to improve the effectiveness of weight loss interventions. An important goal for weight management is to increase total daily energy expenditure (TDEE) by increasing the energy expended in physical activity (total daily activity thermogenesis, TDAT). The two components of TDAT are exercise activity thermogenesis (EAT) and non-exercise activity thermogenesis (NEAT). When exercise is prescribed for weight loss, the expectation is that TDAT and total TDEE will increase by the amount of EAT. However, several studies have demonstrated that the increase in TDAT and TDEE, and consequently weight loss, is less than expected when EAT is increased, even when no changes in energy intake occur. This raises the possibility that a decrease in NEAT compensates for the increased EAT. This question has not been adequately addressed because of the inability to accurately measure NEAT. Through technological innovations, we now have the means of accurately measuring NEAT. There is growing interest in the role of walking in body weight regulation. Walking is an attractive alternative to structured endurance exercise programs because it can be performed intermittently at lower intensities &may, therefore, enhance tolerability &adherence. The primary aim of this proposed study is to determine how TDAT and TDEE are regulated in obese humans attempting to lose weight and maintain weight loss through exercise. Two exercise interventions will be compared;a structured aerobic exercise program and a walking program. EAT will be verified, and NEAT, TDAT, and TDEE will be measured using state-of-the-art technologies. It is hypothesized that the walking program will have more favorable effects on TDAT and TDEE. A secondary aim of the proposed study is to determine whether those individuals that better maintain TDAT and TDEE have better improvement in body weight, body fat, body fat distribution, and metabolic parameters. If our hypotheses are correct, then data from the proposed study could be used to improve the effectiveness of exercise-based weight loss and weight loss maintenance programs.